1. Preheat oven to 350 degrees F. Coat two baking sheets with cooking spray or line with parchment paper and set aside.

2. In a large mixing bowl, use an electric mixer on medium speed to mix butter and brown sugar until well blended, about 3 minutes. Add eggs, one at a time, mixing until smooth after each egg.

3. In a separate bowl, stir together flour, baking soda and salt; set aside. Add half the flour mixture to the butter mixture and mix on medium speed until smooth, scraping down mixing bowl with a rubber spatula. Add half the buttermilk mixture and mix again on medium speed until smooth and slightly fluffy in texture. Repeat with remaining flour and buttermilk, and mix until smooth. Batter will be thick and slightly springy when done.

4. Drop 2-Tbsp. portions of batter onto prepared baking sheets, leaving 2 inches between each portion to allow for spreading. Bake 11 to 13 minutes, until puffed and set but still soft when touched lightly with fingertips. Let cakes cook for 3 minutes on baking sheets before transferring them to wire racks to finish cooling, about 15 to 20 minutes. Repeat with remaining batter.

5. Prepare filling while cakes are baking and cooling. In a large mixing bowl, sift together cocoa powder and confectioners' sugar and set aside. In a large mixing bowl,use an electric mixer on medium speed to cream butter. Add milk, vanilla extract, salt and half the cocoa mixture. Mix on low speed first to combine, then on high until smooth. Add remaining cocoa mixture and mix again. Scrape down sides with a rubber spatula.

6. Add marshmallow creme and mix on medium high until filling is smooth and fluffy, about 3 to 4 minutes.

7. Assemble whoopie pies. Spoon filling onto the flat sides of half the cakes, dividing it evenly. Top with remaining cakes, flat side against the filling, rounded side up. Serve immediately, or wrap and store at room temperature for up to two days, or in freezer for up to two months.